Method of metal recovery using electrochemical cell

Abstract

A high performance method for recovery of metal from aqueous solutions is carried out using an electrochemical cell having a cathode assembly that includes a nonporous support member, a primary cathode, and a nonconductive or conductive porous material covering the primary cathode. An anode is spaced apart from the cathode assembly. Fluid is caused to flow through the porous material to the primary cathode, through openings or fluid collection channels in the nonporous support member, and uniformly out of the cell. Uniform and efficient deposition of metal is accomplished over the entire cathode assembly because of modulation of fluid flow and increased mass transfer.

We claim: 
     
       1. A method for recovering a metal from an aqueous solution containing ions of said metal, said method comprising the steps of: A) contacting an electrochemical cell with said aqueous solution, said electrochemical cell comprising: (a) a cathode assembly comprising: a nonporous support member having uniformly distributed fluid openings or fluid collection channels to provide a uniform fluid pressure drop in said cathode assembly, a conductive primary cathode in intimate contact with said support member, and   at least one conductive or semi-conductive porous secondary cathode covering said primary cathode, and     (b) an anode spaced from said cathode assembly,   said contacting being carried out by causing said aqueous solution to flow through said porous material to said primary cathode, and uniformly through said fluid openings or along said channels of said support member at a rate of from about 10 to about 1000 liters/min/m 2  of primary cathode surface, and     B) applying a negative potential to said cathode assembly.   
     
     
       2. The method of claim 1 wherein said conductive or semi-conductive porous secondary cathode is in contact with said primary cathode. 
     
     
       3. The method of claim 2 wherein said porous secondary cathode is composed of a carbon felt. 
     
     
       4. The method of claim 1 wherein said cathode assembly has a circular geometry, and said anode is coaxially disposed around said cathode assembly. 
     
     
       5. The method of claim 1 wherein said cathode assembly has a planar geometry. 
     
     
       6. The method of claim 1 wherein said support member is composed of a nonconductive material. 
     
     
       7. The method of claim 6 wherein said support member is composed of a ceramic, thermoplastic polymeric material or a thermoset polymeric material. 
     
     
       8. The method of claim 1 wherein said primary cathode is composed of a cermet, ferrous metal, conductive non-ferrous metal or conductive carbon material. 
     
     
       9. The method of claim 8 wherein said primary cathode is composed of a corrosion-resistant stainless steel. 
     
     
       10. The method of claim 1 wherein said cathode further comprises a nonconductive porous barrier material comprising polypropylene, polyethylene, polyester, polyamide or cotton, or a blend of two or more of these. 
     
     
       11. The method of claim 10 wherein a conductive porous secondary cathode is on the outside of said nonconductive porous barrier material. 
     
     
       12. The method of claim 1 wherein said cathode assembly and said anode are spaced apart from about 0.1 to about 20 cm. 
     
     
       13. The method of claim 1 wherein said anode is composed of a cermet, ferrous metal, conductive non-ferrous metal or a conductive carbon material. 
     
     
       14. The method of claim 1 wherein said aqueous solution flow rate is from about 50 to about 800 liters/min/m 2 . 
     
     
       15. The method of claim 1 wherein said fluid openings in said support are of equivalent size and evenly distributed in said support. 
     
     
       16. The method of claim 1 carried out at from about 0.01 to about 0.2 Amp/cm 2  current density. 
     
     
       17. The method of claim 1 wherein said applied negative potential is at least that required for reduction of said metal to be recovered, and the initial electrochemical cell voltage is at least 0.5 volts. 
     
     
       18. The method of claim 1 wherein said metal ions are present in said aqueous solution in a concentration of at least 1 ppm. 
     
     
       19. The method of claim 1 wherein said metal ions are present in said aqueous solution in a concentration of at least 50 ppm. 
     
     
       20. The method of claim 1 wherein said metal ions are silver, gold, copper, lead, tellurium, platinum, palladium or nickel ions. 
     
     
       21. The method of claim 20 wherein said metal ions are silver ions. 
     
     
       22. The method of claim 1 carried out by contacting two or more of said electrochemical cells operated in a series or parallel arrangement with said aqueous solution. 
     
     
       23. The method of claim 1 wherein said aqueous solution is a photographic processing solution. 
     
     
       24. The method of claim 22 wherein said aqueous solution is a photographic fixing or bleach-fixing solution. 
     
     
       25. A method for recovering a metal from an aqueous solution containing ions of said metal, said method comprising the steps of: A) contacting an electrochemical cell with said aqueous solution, said electrochemical cell comprising: (a) a cathode assembly comprising: a nonporous support member having uniformly distributed fluid openings or fluid collection channels to provide a uniform fluid pressure drop in said cathode assembly,   conductive primary cathode in intimate contact with said support member, and   at least one porous material covering said primary cathode such that the deposited metal is predominantly collected in and on the porous material, and     (b) an anode spaced from 0.1 to 20 cm from said cathode assembly to allow the deposited metal to grow outwardly toward the anode and such that the deposited metal can be readily removed from the rest of the cathode assembly with the porous material,   said contacting being carried out by causing said aqueous solution to flow through said porous material to said primary cathode, and uniformly through said fluid openings or along said channels of said support member at a rate of from about 10 to about 1000 liters/min/m 2  of primary cathode surface, and     B) applying a negative potential to said cathode assembly.   
     
     
       26. A method for recovering a metal from an aqueous solution containing ions of said metal, said method comprising the steps of: A) contacting an electrochemical cell with said aqueous solution, said electrochemical cell comprising: (a) a fluid distribution plate for uniformly distributing entering fluid to an interelectrode gap within the cell;   (b) a cathode assembly having a circular geometry comprising: a nonporous support member having fluid collection channels, which support member is hollow inside such that the volume of fluid required to fill the cell during operation is only that volume of the interelectrode gap, a conductive primary cathode in intimate contact with said support member, and at least one porous material covering said primary cathode,     (c) an annular anode coaxially disposed around and spaced from said cathode assembly to form the interelectrode gap through which fluid is forced during operation, and   (d) a fluid collection manifold and fluid outlet for allowing said fluid flowing along said collection channels to exit the cell,   contacting being carried out by causing said aqueous solution to flow through said porous material to said primary cathode, and uniformly through said fluid openings or along said channels of said support member at a rate of from about 10 to about 1000 liters/min/m 2  of primary cathode surface, and     B) applying a negative potential to said cathode assembly.